The present invention relates to an electric power steering apparatus.
Conventionally, an electric power steering apparatus (EPS) provided with a motor as a driving source applies a motor torque as an assist force to a steering system by rotating a steering shaft. In general, in such an EPS, the motor is coupled to the steering shaft via a speed reducing mechanism (for example, a worm and wheel mechanism), which includes first and second gears meshed with each other. For example, according to the configuration disclosed in Japanese Laid-Open Patent Publication No. 2004-291718, rotation of the motor is reduced by the speed reducing mechanism, and is transmitted to the steering shaft.
Most EPS apparatuses include a torque sensor 93 as shown in FIG. 16. The torque sensor 93 detects a steering torque based on a torsion angle of a torsion bar 92 provided in a steering transmitting system (a steering shaft 91), which couples the steering wheel and steerable wheels. FIG. 16 shows a twin resolver type torque sensor widely employed as an EPS torque sensor. This type of a torque sensor detects a torsion angle of the torsion bar 92 by a pair of angle sensors 94a, 94b (resolvers) provided on the ends of the torsion bar 92. The EPS applies an assist force in a direction to reduce a steering reaction force based on the steering torque detected by the torque sensor.
In the above-mentioned EPS, as the motor rotates in order to reduce the steering reaction force, a rattling noise is generated at meshing parts of gears configuring the speed reducing mechanism. For example, in the case that a stress is applied to the steerable wheels such as when traveling on a rough road surface, the steering shaft is rotated by the stress. The torque sensor detects the torque based on the stress applied to the steerable wheels as the steering reaction force. At this time, the motor rotates to apply an assist force in a direction to cancel the steering reaction force. Thus, the rattling noise is generated in the speed reducing mechanism.
More specifically, as shown in FIG. 17, when a stress is applied to the steerable wheels, a first gear (reduction gear) 95, which rotates with the steering shaft, and a second gear (motor gear) 96, which is rotated by the motor, are rotated in opposite directions. As a result, teeth 95a, 96a of the gears 95, 96, which are meshed with each other, collide one another. Furthermore, the stress applied to the steerable wheels remains in the steering system as vibration. Thus, since the first and second gears 95, 96 repeatedly collide with each other while being inverted, the rattling noise is transmitted outside.